A single phase magnetite powder as one of the magnetic oxides has been utilized in a wide range of technical fields, e.g., those of magnetic fluids, electric resistance elements and toners and carriers for electrophotography. It is highly desired in the art to mass-produce a single phase magnetite powder exhibiting any predetermined saturation magnetization at a lowered cost.
The following three general methods are known for producing such a magnetite, and it is know that they have respective drawbacks as described below.
(1) Wet Process: an aqueous solution of Fe.sup.2+ +2Fe.sup.+3 is rendered alkaline and coprecipitated. PA1 (2) Dry Process: hematite is heated in either hydrogen and carbon monoxide or steam to thereby be reduced. PA1 (3) Process in which natural magnetite is pulverized. PA1 Ca compound: 1.43 to 64.0% by weight (in terms of Ca) PA1 Sintering temperature: 1200.degree. to 1450.degree. C. PA1 Ca compound: 1.43 to 64.0% by weight (in terms of Ca) PA1 Sintering temperature: 550.degree. to 1450.degree. C. PA1 Hematite: 0.0 to 79.0% by weight PA1 Magnetite: 9.8 to 98.8% by weight PA1 Ti compound: 1.2 to 52.7% by weight (in terms of Ti) Sintering temperature: 1200.degree. to 1450.degree. C. PA1 Ti compound: 1.2 to 52.7% by weight PA1 Sintering temperature: 550.degree. to 1450.degree. C. PA1 Sn compound: 0.8 to 76.0% by weight PA1 Sintering temperature: 1200.degree. to 1450.degree. C. PA1 Sn compound: 0.8 to 76.0% by weight PA1 Sintering temperature: 550.degree. to 1450.degree. C. PA1 Si compound: 0.5 to 65% by weight PA1 Sintering temperature: 1200.degree. to 1450.degree. C. PA1 Si compound: 35 to 99.5% by weight PA1 Sintering temperature: 550.degree. to 1450.degree. C. PA1 Mg compound: 0.20 to 18.50% by weight PA1 Sintering temperature: 550.degree. to 1500.degree. C. PA1 Mg compound: 0.20 to 26.20% by weight PA1 Sintering temperature: 550.degree. to 1500.degree. C. PA1 Addition of manganese: 0.20 to 10.0% by weight PA1 Mg compound: 0.20 to 26.20% by weight PA1 Sintering temperature: 550.degree. to 1500.degree. C. PA1 Addition of manganese: 0.20 to 10.0% by weight PA1 Mg compound: 0.20 to 26.20% by weight PA1 Sintering temperature: 550.degree. to 1500.degree. C.
Drawbacks: production cost is high and it is not suitable for mass production, although fine particles each having a relatively high purity are readily obtained by coprecipitation. PA2 Drawbacks: the reaction is performed in a relatively highly reducing atmosphere, e.g., either high pressure steam or hydrogen and carbon monoxide, so that danger accompanies the mass production, and that large scale equipment is required. PA2 Drawbacks: a natural mineral is used as a starting material, so that it is difficult to stably produce fine quality powder for a prolonged period of time.
The magnetite powders produced by the above conventional processes have a drawback in that their saturation magnetization values are so high as compared with those of the conventional spinel ferrite that it is unfeasible to attain control of the saturation magnetization by composition regulation with the result that the conventional magnetite powders cannot be employed in use in which the saturation magnetization values inherent in magnetite powder are unacceptable. Magnetite powder having a saturation magnetization value inherent therein (fixed value specified in the second experimental example described later, e.g., up to 92 emu/g) has only partly been employed in use in carriers for electrophotographic development and the like. Moreover, the above magnetite powder having the saturation magnetization value inherent therein as it is cannot replace ferrite or the like in uses in which it has been employed, and, when the replacement is to be made, it is required to alter the circuit or device to be employed disadvantageously.
Taking into account the advantages and disadvantages of the above processes, various proposals have been made with respect to the production of magnetite powder for use in magnetic toners for electrophotography, etc.
For example, Japanese Patent Publication Nos. 238,580/1987, 39,498/1990 and 51,505/1990 disclose processes for producing magnetite useful as a carrier component of an electrophotographic developer, especially a two-component developer composed of a toner and a carrier. Each of these processes comprises providing magnetite powder (or globular magnetite particles) as a starting material, granulating the same into globular grains through, for example, mixing with a binder, followed by heating (sintering), and applying a resin coating to the resultant globular magnetite particles according to the conventional technique.
The use of a material called soft ferrite as carrier particles is known (U.S. Pat. No. 3,929,657, etc.). In this connection, it is known that the carrier particles each composed of the above ferrite not only are excellent in magnetic properties but also do not require a resin coating layer, so that their durability is excellent. Nothing that ferrite particles have resistance variations depending on the control of sintering atmosphere, even with the same composition, attempts have been made to enlarge resistance variation ranges by changing the sintering atmosphere. As an example thereof, there can be mentioned an attempt described in Japanese Patent Publication No. 37782/1987. This comprises providing an Mg ferrite as a starting material and increasing the amount of ferric oxide (Fe.sub.2 O.sub.3) to more that 53% by mole so as to enlarge its resistance variation range.